New cop to discourage context with only a single test

[fatkodima]

Relevant to #2000.

When the context block has only a single test, it makes little sense to actually use the context block and making the code unnecessary indented and a little more complicated.

# bad
context 'when condition' do
  it 'tests something' do
  end
end

# good
it 'tests something when condition' do
end